Tracing Trends and Changes of Drought in Hydrosystems (TrenDHy)
Final Report Abstract
Drought is a natural hazard with severe effects on low flows from mountain headwaters to lowlands. Global change will affect headwater catchments with their important water storages such as glaciers, lakes, wetlands, or productive groundwater in valleys and their subsequent ability to sustain river flow and water supply through meteorological droughts. TrenDHy investigated two hydrological systems: glacierized headwaters that are deglaciating and upland headwaters and lowlands where baseflow stems mainly from groundwater. For both systems, multi-catchment datasets with relatively long streamflow records and important catchment or system characteristics were assembled for Germany and for different glacierized regions in the Alps, Norway, Canada. In addition, catchment-scale glacio-hydrological models (HBV-light) and a MODFLOW-based groundwater model at the scale of Germany were set up and structurally improved to serve the project's purpose. Initial empirical analyses of the streamflow records showed a large variation in the trends in these headwater systems over the past decades, especially in the extremes. Filtering and regional averaging revealed distinct regional changes that can be attributed to progressive glacier retreat in the Alps and Canada and beginning declines in Norway. But these changes did not simply follow the commonly assumed 'glacier peak water' trajectory. Glacier cover plays an important but not the only role in the glacier compensation effect of glacierized headwater catchments; Van Tiel et al. (2020) showed that models possibly simplify the relation too much. Nevertheless, a detailed analysis of warm and dry events suggested that the level of drought-compensation by snow and glacier melt has already been decreasing with time. To explain the substantial variation among catchments, additional characteristics about the glaciers as well as the non-glacierized part of the catchment will be necessary. Groundwater and in particular baseflow trends in non-glacierized upland headwaters are also highly variable. Results for any trend analysis depend on the choice of time series due to long-term fluctuations. Climate sensitivity can be estimated by a typical lag time between climate anomalies and groundwater drought though. Hellwig et al. (2021) built a stress-testing framework on that idea with the help of a large-scale groundwater mode allowing for example to estimate the sensitivities of known drought events to altered antecedent conditions. Groundwater sensitivities across Germany are diverse, but a major threat are combinations of long-term precipitation deficits with summer drought events as experienced in recent years as these may cause very widespread effects. Aspects of simultaneousness, spatial extent and the compound hazard with heatwaves should therefore be considered in future stress tests. Overall, the modelling experiments carried out within the project illustrate that models simplify cause and effect considerably and underestimate the variability among headwater catchments that sustain streamflow from a range of catchment stores that may compensate deficits at different time scales. Nevertheless, models are useful tools to develop characteristic change trajectories for different hydrological systems and a way forward is to combine long-term changes with eventbased stress test approaches. These can add valuable information for adaptation to an increasing drought hazard that may put central Europe's water resources at risk in the future.
Publications
- (2018): An assessment of trends and potential future changes in groundwater-baseflow drought based on catchment response times. Hydrol. Earth Syst. Sci., 22, 6209-6224
Hellwig, J. & Stahl, K.
(See online at https://doi.org/10.5194/hess-22-6209-2018) - (2020) Large-Scale Assessment of Delayed Groundwater Responses to Drought. Water Resour. Res., 56
Hellwig J., de Graaf I. E. M., Weiler M., Stahl K.
(See online at https://doi.org/10.1029/2019WR025441) - (2020). The compensating effect of glaciers: Characterizing the relation between interannual streamflow variability and glacier cover. Hydrol. Proc., 34(3), 553-568
Van Tiel, M., Kohn, I., Van Loon, A. F., & Stahl, K.
(See online at https://doi.org/10.1002/hyp.13603) - (2020): Glacio-hydrological model calibration and evaluation, WIREs Water, 7, e1483
Van Tiel, M., Stahl, K, Freudiger, D., Seibert, J.
(See online at https://doi.org/10.1002/wat2.1483) - (2021): Groundwater and baseflow drought responses to synthetic recharge stress tests, Hydrol. Earth Syst. Sci., 25, 1053–1068
Hellwig, J., Stoelzle, M., and Stahl, K.
(See online at https://doi.org/10.5194/hess-25-1053-2021) - (2021): Hydrological response to warm and dry weather: do glaciers compensate?, Hydrol. Earth Syst. Sci., 25, 3245–3265
Van Tiel, M., Van Loon, A. F., Seibert, J., and Stahl, K.
(See online at https://doi.org/10.5194/hess-25-3245-2021)
